Spinal implant systems can be engaged to or along one or more vertebrae of the spinal column for the treatment of various spinal conditions. Fasteners can be provided to secure the implant to a particular location along the spinal column. The implants can be provided to stabilize the spinal column for treatment, either by fixing the spinal column or by permitting at least some motion of the stabilized motion segments.
Multi-axial and uni-axial screws have been employed for securing elongated implants, such as rods, along one or more motion segments of the spinal column. Bolts, screws, and hooks are typically secured to the vertebrae for connection to the supporting rod. These vertebral anchors must frequently be positioned at various angles due to the anatomical structure of the patient, the physiological problem being treated, and the preference of the physician. It is difficult to secure connections between the spinal support rod and the vertebral anchors at all the various angles and elevations that are required, especially where there are different distances between the rod and bolts and where these components are located at different heights on the patient.
In the field of orthopedic stabilization and specifically, spinal fixation of vertebrae, there exist numerous devices designed to allow the surgeon to place screws, helical spikes, pegs, or other bone attachment means within bone tissue, and are adapted to allow the attachment of rods, plates, or other longitudinal support members in order to stabilize one vertebrae to adjacent vertebrae, and thereby, assist in the fusion or healing process of adjacent vertebrae. As mentioned earlier, these devices may be mono-axial which require a 90-degree connection between the axis of the screw and the axis of the rod, or can be poly-axial or variable-angle, thus allowing a rod/screw connection of an angle other than perpendicular. Examples of the aforementioned can be found in U.S. Pat. Nos. 6,280,442 and 5,474,555. Further, there exist some devices that allow both poly-axial positioning as well as variable dorsal height positioning (see U.S. Pat. Nos. 6,626,906 and 5,885,285). There have existed bone screws that contain a bulbous or cylindrical body portion that engage the bone opening and help secure the attachment of the bone screw to the bone (see U.S. Pat. No. 4,854,311).
However, there is a need for a connection assembly between a spinal rod and a vertebral anchor that allows the surgeon to fix the desired elevation between the rod and the bone anchor as well as fix the desired angle between the anchor and the spinal rod. This need also encompasses the need for minimizing the profile and bulk of any of the components used to engage the bone screw to the spinal rod in a variety of angular orientations.